In recent years, various systems capable of tracking objects using, for example, an ITV camera, to perform continuous monitoring or obtain detailed information have been commercialized for use in large-scale facilities, such as airports and factories, security equipment employed in electrical power plants or water-supply corporations associated with lifeline, and traffic information support systems, such as ITSs. These tracking systems have a structure contrived to be used not only as ground equipment, but also as systems installed on a moving platform such as a vehicle, ship or airplane. To this end, the structure is made compact and resistance against vibration. More specifically, the tracking systems perform disturbance suppression against vibration and swinging. Further, to enable the tracking systems to sequentially track a plurality of targets, it has come to be important to increase tangential velocity of the systems to cause them to be directed to the targets in a short time.
In such a conventional moving object image tracking system as the above, to track a target that moves in all directions, a gimbal structure is often employed. The gimbal structure needs to have at least two axes. In a biaxial gimbal, when a target passes through or near the zenith, it is necessary for its Az axis to instantly shift its orientation from the front to the rear, namely, to instantly rotate through about 180 degrees. However, since there is a limit in motor torque, this operation is difficult to perform, with the result that such a phenomenon as a so-called gimbal lock, in which continuous tracking becomes impossible, will occur. Thus, in the biaxial gimbal structure, the gimbal cannot be oriented toward the zenith or its neighborhood, which makes it difficult to continuously track a target in all directions.
To overcome this disadvantage, image tracking systems having a triaxial gimbal structure exist. In the triaxial gimbal structure, the freedom of motion is increased, and motion is divided into Az axis motion and xEL axis motion in order to avoid excessive angular velocity, thereby enabling the gimbal to continuously track a target in all directions within an allowable gimbal rotation range and without gimbal lock.
Other types of conventional tracking mechanisms, which employ no gimbal structure, have been proposed. In these mechanisms, a frictional rolling motion mechanism rotates a spherical casing in all directions.
Further, a convey apparatus has been proposed as a mechanism for rotating a spherical movable body utilizing friction. In this apparatus, it is difficult to reduce the size, and a control rule for tracking a target is complex. For instance, if the apparatus has a triaxial gimbal structure, the number of driving means, such as motors, increases, which makes reduction of size and cost difficult. Further, since the apparatus is provided with, for example, a camera, the load inertia of the xEL axis is high, which increases the possibility of axial interference between the Az axis and the xEL axis. This is a problem peculiar to the triaxial structure. Although it is possible to reduce the angular velocity of the Az axis using a redundancy axis, the Az axis requires a higher angular velocity than the other axes, the required driving torque will be inevitably increased.
In the systems with no conventional gimbal structure, there is no problem of gimbal lock. In this case, however, it is difficult to achieve automation of the systems as moving object image tracking systems. For instance, it is necessary to manually drive a spherical body employed therein by remote control until a target enters the image screen of a camera used therein. Further, these systems do not have any element for acquiring information concerning the orientation of the camera. Because of the above, it is difficult for the systems to realize automatic tracking of targets using information obtained from images of the targets. In addition, in these systems, wireless communication is performed with, for example, the camera in the spherical body, which inevitably limits the operation period of the camera.
Moreover, although the above-mentioned conventional mechanism for driving the spherical body is applicable to, for example, a moving apparatus, it cannot easily be applied to moving object image tracking systems. For example, in the mechanism, the spherical body is moved, with a table, installed therein, kept horizontal. This makes it difficult to orient the camera in an arbitrary direction.